Superheterodyne transmitters, zero IF (Intermediate Frequency) transmitters and DIF (Digital Intermediate Frequency) transmitters are the most commonly used transmitters. Superheterodyne transmitters are capable of realizing two times of up-conversion by using analog devices, and the greatest advantage of superheterodyne transmitters is excellent selectivity, namely, the ability to process and select small signals in the presence of strong interference signals. Superheterodyne transmitters have been widely used in various wireless communications systems due to its sophisticated design and continuously improving performance and integration.
A zero IF transmitter is an improvement of the superheterodyne transmitter based on the zero IF technology. A zero IF transmitter has the same analog IF module with a superheterodyne transmitter, but a zero IF transmitter omits the process of analog IF and directly performs up-conversion, so that the architecture is simplified. FIG. 1 is a schematic diagram of the structure of a zero IF transmitter in the prior art. As shown in FIG. 1, the zero IF transmitter is comprised of two orthogonal digital-to-analog converters, two orthogonal baseband filters, a single sideband up-conversion mixer and a power amplifier. The operation principle of the zero IF transmitter is as follows: digital baseband IQ signals (I_DAC<N: 1> and Q_DAC<N: 1>) are converted to analog IQ signals by the digital-to-analog converters; the analog IQ signals are passed through the baseband filters and then are mixed with two orthogonal local oscillator signals (LO_Q and LO_I) and are finally added together, so that the analog IQ signals are converted to analog RF modulated signals and are outputted (TX_OUT) via the power amplifier. As shown in FIG. 2, the up-conversion mixer is comprised of a high linearity voltage-to-current conversion circuit and an up-conversion mixer core circuit, wherein the high linearity voltage-to-current conversion circuit is used for converting the baseband signals that have passed through the baseband filters (BB_INP and BB_INN) to current signals; the up-conversion mixer core circuit is used for mixing the current signals outputted by the high linearity voltage-to-current conversion circuit with the LO signals so as to output high linearity frequency-mixed signals (MIX_OP and MIX_ON).
Nevertheless, the baseband filter and the up-conversion mixer are both employed in the transmitter of such structure, which has the disadvantages of occupying a large chip area and generating a high current, therefore, an improved technical solution is needed to solve this problem.